1. Field of the Invention
The present invention relates to an apparatus for monitoring optical frequencies using an arrayed-waveguide grating (demultiplexer) in wavelength division multiplexed optical communication networks. More particularly, the invention relates to a wavelength division multiplexed apparatus for monitoring the optical frequencies utilizing an arrayed-wavequide grating and pilot tone signals applied on optical signals in order to enhance the efficiency of operation, administration, and maintenance of wavelength division multiplexed optical communication networks.
In Wavelength Division Multiplexing (WDM) optical communication systems, a plurality of transmission lasers with different wavelengths are multiplexed on a single optical fiber so that the transmission capacity per fiber is enhanced, endowing the network with broadband and high speed.
However, since the optical frequencies of optical signals may change due to the system aging, temperature change, etc. and the optical frequency change on each channel may have a large impact on the system performance by causing a change in channel output power and crosstalk with the adjacent channels due to the different frequency-dependent transmission properties of different optical elements, there is always needed to monitor optical frequencies of each channel in wavelength division multiplexed systems.
2. Description of the Related Art
There has been proposed a technique in the prior art for monitoring optical frequencies of multiple channels in which the wavelength crossover properties of an arrayed-waveguide grating is utilized to monitor optical frequency of optical signals by passing the optical signals through the arrayed-wavequide grating.
However, this technique in the prior art has an economic disadvantage that two arrayed-waveguide grating ports and two photodetectors are required to monitor one optical signal because two optical signals carried on one port of the arrayed-waveguide grating cannot be differentiated.
Another frequency monitoring technique in the prior art applicable to wavelength division multiplexed optical transmission systems uses frequency tunable passband filters such as acousto-optic tunable filters or temperature tunable etalon filters that are able to vary the frequency of the passband.
However, this technique not only requires a delicate tuning mechanism though they allow a simple configuration, but has insufficient reliability and resolution for dense WDM optical systems.
There has been proposed still another simple technique which uses a fixed Fabry-Perot etalon filter and pilot tones for monitoring optical frequencies of multiple channels without using tunable filters and monitors the optical frequencies of the optical signals falling in the vicinity of the resonance frequency of the Fabry-Perot etalon filter.
However, this technique has a disadvantageous degradation in the monitoring performance after passing an optical amplifier due to the cross-gain modulation of erbium-doped optical fiber amplifier (EDFA).